Referring first to FIGS. 1 and 2, the mitral valve 50 controls the flow of blood between the left atrium 52 and the left ventricle 54 of the human heart. After the left atrium 52 receives oxygenated blood from the lungs via the pulmonary veins, the mitral valve 50 permits the flow of the oxygenated blood from the left atrium 52 into the left ventricle 54. When the left ventricle 54 contracts, the oxygenated blood that was held in the left ventricle 54 is delivered through the aortic valve 56 and the aorta 58 to the rest of the body. Meanwhile, the mitral valve should close during ventricular contraction to prevent any blood from flowing back into the left atrium.
When the left ventricle contracts, the blood pressure in the left ventricle increases substantially, which serves to urge the mitral valve closed. Due to the large pressure differential between the left ventricle and the left atrium during this time, a large amount of pressure is placed on the mitral valve, leading to a possibility of prolapse, or eversion of the leaflets of the mitral valve back into the atrium. A series of chordae tendineae 62 therefore connect the leaflets of the mitral valve to papillary muscles located on the walls of the left ventricle, where both the chordae tendineae and the papillary muscles are tensioned during ventricular contraction to hold the leaflets in the closed position and to prevent them from extending back towards the left atrium. This helps prevent backflow of oxygenated blood back into the left atrium. The chordae tendineae 62 are schematically illustrated in both the heart cross-section of FIG. 1 and the top view of the mitral valve of FIG. 2.
A general shape of the mitral valve and its leaflets as viewed from the left atrium is shown in FIG. 2. Commissures 64 are located at the ends of the mitral valve 50 where the anterior leaflet 66 and the posterior leaflet 68 come together. Various complications of the mitral valve can potentially cause fatal heart failure. One form of valvular heart disease is mitral valve leak or mitral regurgitation, characterized by abnormal leaking of blood from the left ventricle through the mitral valve back into the left atrium. This can be caused, for example, by dilation of the left ventricle causing the native mitral leaflets to not coapt completely, resulting in a leak, by damage to the native leaflets, or weakening of (or damage to) the chordae tendineae and/or papillary muscles. In these circumstances, it may be desirable to repair the mitral valve or to replace the functionality of the mitral valve with that of a prosthetic heart valve.
With respect to valve replacement, while open surgical procedure options are more readily available, there has been much less development in terms of commercially available ways to replace a mitral valve through catheter implantation and/or other minimal or less invasive procedures. In contrast, the field of transcatheter aortic valve replacement has developed much more and has gained widespread success. This discrepancy stems, in part, from replacement of a mitral valve being more difficult than aortic valve replacement in many respects, for example, due to the non-circular physical structure of the mitral valve, its sub-annular anatomy, and more difficult access to the valve. Due to the successes in the development of transcatheter aortic valve technology, it could be beneficial to use the same or similar circular valve prostheses for mitral valve replacements.
One of the most prominent obstacles for mitral valve replacement is effective anchoring or retention of the valve at the mitral position, due to the valve being subject to a large cyclic load. As noted above, another issue with mitral valve replacement is the size and shape of the native mitral annulus, as can be seen in FIG. 2. Aortic valves are more circular or cylindrical in shape than mitral valves. Also, the mitral and tricuspid valves are both larger than the aortic valve, and more elongate in shape, making them more difficult and unconventional sites for implanting a replacement valve with a generally circular or cylindrical valve frame. A circular prosthetic valve that is too small can result in leaking around the implant (i.e., paravalvular leakage) if a good seal is not established around the valve, while a circular prosthetic valve that is too large can stretch out and damage the narrower parts of the native mitral annulus. Further, in many cases, the need for aortic valve replacement arises due, for example, to aortic valve stenosis, where the aortic valve narrows due to calcification or other hardening of the native leaflets. Therefore, the aortic annulus generally forms a more compact, rigid, and stable anchoring site for a prosthetic valve than the mitral annulus, which is both larger than the aortic annulus and non-circular. Instances of mitral valve regurgitation are unlikely to provide such a good anchoring site. Also, the presence of the chordae tendineae and other anatomy at the mitral position can form obstructions that make it much more challenging to adequately anchor a device at the mitral position.
Other obstacles to effective mitral valve replacement can stem from the large cyclic loads the mitral valve undergoes and the need to establish a sufficiently strong and stable anchoring and retention. Also, even a slight shift in the alignment of the valve can still lead to blood flow through the valve or other parts of the heart being obstructed or otherwise negatively impacted.